This invention relates to a polyester film for use in a magnetic recording medium, and more specifically to a polyester film useful for production of a magnetic recording medium excellent in runnability, electromagnetic conversion characteristics and storage stability.
As a high-density magnetic recording medium, a magnetic recording medium of a ferromagnetic metallic thin film is known which is formed by physical sedimentation such as vacuum deposition or sputtering, or by plating. For example, a magnetic tape having Co deposited thereon (Japanese Laid-open Patent Application No. 147,010/1979) and a vertical magnetic recording medium using a Co-Cr alloy (Japanese Laid-open Patent Application No. 134,706/1977) are known. Such a metallic thin film formed by a thin film forming means such as deposition, sputtering or ion plating has advantages that though said film has a thickness as low as less than 1.5 micrometers, it can provide performance equal to or higher than that of a coated magnetic recording medium (a magnetic recording medium obtained by mixing a magnetic powder with an organic polymer binder and coating the mixture on a non-magnetic support) having a magnetic recording layer more than 3 micrometers in thickness.
Magnetic characteristics such as coercivity Hc and a squareness ratio of a hysteresis loop, which are static characteristics of the magnetic recording medium are thought to be less dependent on the surface condition of the non-magnetic support being used. As an example relying on such a thought, a Co-Cr multilayered structure formed by vacuum deposition is disclosed in U.S. Pat. No. 3,787,327.
In the metallic thin film-type magnetic recording medium, however, the thickness of the metallic thin film formed on the surface of the non-magnetic support is low, and the surface condition (surface unevenness) of the non-magnetic support is developed as such, as unevenness of the surface of the magnetic recording layer, which becomes a cause of noise.
From the aspect of noise, it is advisable that the smoother the surface of the non-magnetic support, the better. On the other hand, from the aspect of handling such as winding or unwinding of a base film, the surface of the base film is required to be rough because if the film surface is smooth, the sliding property between the films is poor causing a blocking phenomenon. Thus, from the aspect of electromagnetic characteristics, the surface of the non-magnetic support is required to be smooth, whereas from the aspect of handlability, the film surface is required to be rough. Therefore, a base film that meets both the contradictory qualities at the same time has been demanded.
Moreover, runnability of the metallic thin film surface is a serious problem when the metallic thin film magnetic recording medium is actually used. A usual coated magnetic recording medium formed by mixing a magnetic powder with an organic polymer binder and coating the mixture on a base film can improve runnability of the magnetic surface by dispersing a lubricant in the binder. However, in case of the metallic thin film magnetic recording medium, such a measure cannot be taken; it is very hard to stably maintain runnability, and especially runnability at a high temperature and high humidity is poor.
In order to remedy the defects, it is proposed in Japanese Patent Publication No. 30,105/1987 that very small protuberances be formed on the film surface using fine particles, a water-soluble resin and a silane coupling agent. Further, it is proposed in Japanese Patent Publication No. 30,106/1987 and Japanese Laid-open Patent Application No. 292,316/1984 that very small protuberances are formed on the film surface using fine particles and a water-soluble resin. In the above proposals, however, the fine particles are present in the trapezoidal protuberances of the water-soluble resin; it is not that the fine particles are uniformly present on the film surface. Still further, it is proposed in Japanese Patent Publication No. 34,456/1989 that a discontinuous film of a water-soluble polymer and fine particles forming a higher protuberance than said film be independently adhered to the film surface. However, according to such process, since the discontinuous film and the fine particles are not uniformly dispersed, uniformity of the film surface is poor.